Journal Archive

Platinum Metals Rev., 2004, 48, (1), 16

Platinum Group Metal Chalcogenides

THEIR SYNTHESES AND APPLICATIONS IN CATALYSIS AND MATERIALS SCIENCE

  • Sandip Dey
  • Vimal K. Jain*
  • Novel Materials and Structural Chemistry Division, Bhabha Atomic Research Centre,
  • Mumbai 400085, India
  • Email: jainvk@apsara.barc.ernet.in

Article Synopsis

Some salient features of platinum group metal compounds with sulfur, selenium or tellurium, chalcogenides, primarily focusing on binary compounds, are described here. Their structural patterns are rationalised in terms of common structural systems. Some applications of these compounds in catalysis and materials science are described and emerging trends in designing molecular precursors for the syntheses of these materials are highlighted.

Chalcogenides are a range of compounds that primarily contain oxygen, sulfur, selenium, tellurium or polonium and which may also occur in nature. The compounds may be in binary, ternary or quaternary form. Platinum group metal chalcogenides have attracted considerable attention in recent years due to their relevance in catalysis and materials science. Extensive application of palladium (∼ 27% of global production in year 2000) in the electronic industry in multilayer ceramic capacitors (MLCCs) and ohmic contacts has further accelerated research activity on platinum group metal chalcogenide materials.

The platinum group metals form several chalcogenides:

  • binary,

  • pseudo-binary for example, Ru1−xOsxS2, NixRu1−xS2, etc., and

  • ternary, such as spinels: M′M2E4 (M′ = Cr, Mn, Fe, Co, Ni, Cu; M = Rh, Ir; E = S, Se, Te) and Tl2Pt4E6 (E = S, Se, Te); MoRuS, etc. These differ in stoichiometry and structures. Several of the chalcogenides occur in nature as minerals, for instance, laurite (RuS2), braggite (PdPt3S4), luberoite (Pt5Se4), Pd6AgTe4, etc. Although there is an extensive literature on the synthesis and structural aspects of bulk platinum group metal chalcogenides, only recently has there been research into their catalytic and electronic properties, and into the preparation of nanoparticles and thin films.

This review intends to cover these emerging aspects of platinum group metal chalcogenides and will consider first a selection of the structures adopted, followed by their catalytic and electronic uses.

Ruthenium and Osmium Chalcogenides

Ruthenium and osmium dichalcogenides of composition ME2 (M = Ru, Os; E = S, Se, Te) are usually prepared by heating stoichiometric quantities of the elements in evacuated sealed ampoules at elevated temperatures (∼ 700°C) (1–3). Single crystals, such as RuS2, see Figure 1, are grown either from a tellurium flux (4) or by chemical vapour transport techniques using interhalogens as transporting agent (5). This latter technique, employing Cl2/AlCl3 as the transport agent, has been exploited, for example, to prepare a low temperature modification of α-RuTe2, which crystallises in the marcasite structure. The orthorhombic space group is Pnnm (No 58) (a = 5.2812(13), b = 6.3943(19), c = 4.0085(13) Å) (6). This low-temperature (marcasite) phase transforms into the pyrite structure at 620°C (7).

Fig. 1

Structure of RuS2 (pyrite structure) (77). The black circles are Ru and the open circles are S

Extensive structural studies both by powder (8–12) and single crystal (13, 14) X-ray measurements on ME2 show that these compounds adopt a pyrite (FeS2) structure, and crystallise in a cubic system with space group Pa3, see Table I. The chalcogenide ions in these structures are tetrahedrally surrounded by another anion (E−) and three metal ions. The metal ions occupy the tetrahedral holes formed by the anion sublattice, see Figure 1.

Table I

Crystallographic Information and Band Gap of Ruthenium/Osmium Chalcogenides

MxEy Crystal system Space group Cell parameter, Å JCPDS-ICDD#File No. Band gap, eV* Shortest M–E and E–E bond lengths, Å
M–E E–E
RuS2 cubic Pa-3 5.610 80–0669 1.22 (opt.)(19) 2.3520(3)(13) 2.1707(8)
RuSe2 cubic Pa-3 5.933 80–0670 0.76 (opt.)(19) 2.4707(2)(13) 2.4532(2)
Ø 0.6 (el.)(14)
RuTe2 cubic Pa-3 6.391 79–0252 0.39 ± 0.01 (opt.)(24) 2.647(8) 2.791
0.25 (el.)(14)
OsS2 cubic Pa-3 5.619 84–2332 ∼ 2.0 (opt.)(14) 2.352(8) 2.210
OsSe2 cubic Pa-3 5.946 73–1693 –
OsTe2 cubic Pa-3 6.397 84–2333 > 0.2 (el.)(14) 2.647(8) 2.826

[i] # Powder diffraction files compiled by JCPDS, International Center for Diffraction Data, U.S.A., 1997

[ii] * Band gap measured from optical (opt.) or electrical resistivity (el.)

Magnetic susceptibility measurements on ME2 indicate diamagnetic behaviour (15). Infrared and Raman spectra of pyrite-type ME2 have been reported (16–19). A comparison of the Raman and IR frequencies shows that the corresponding metal-chalcogen and chalcogen-chalcogen bonds have nearly the same strengths (at least for RuS2 and RuSe2) (17), but the metal-chalcogen bond strength increases in the order Ru < Os (18). Of the five expected absorptions for ME2, the MTe2 compounds display only one phonon peak in the Raman spectra which indicates the metallic behaviour of the tellurides (19). Studies of their optical absorption (15, 20–22), electrical resistivity (15, 20, 23–25) and Hall effect measurements (23–26) have shown that ME2 are indirect band n-type semiconductors.

Rhodium and Iridium Chalcogenides

Rhodium and iridium form a variety of chalcogenides differing in stoichiometry and structural patterns, see Table II. At least four different families of compounds can essentially be identified:

Table II

Crystallographic Information and Band Gaps of Rhodium/Iridium Chalcogenides

MxEy Crystal system Space group Cell parameters JCPDS-ICDD# File No. Band gap, eV
a, Å b, Å c, Å α, ° β, ° γ, °
RhS2 cubic Pa-3 (205) 5.58 – – – – – 03–1210
Rh2S3 orthorhombic Pbcn (60) 8.462 5.985 6.138 – – – 72–0037 ∼ 0.6 (opt)(27)
0.8 (el)
Rh17S15 cubic Pm-3m (221) 9.911 – – – – – 73–1443
Rh3S4 monoclinic C2/m 10.29 (2) 10.67 (1) 6.212 (8) – 107.70 – metallic(34)
Rh3Se4 hexagonal P 7.296 – 10.986 – – – 19–1051
Rh2Se3 orthorhombic Pbcn (60) 8.888 6.294 6.423 – – – 19–1054
RhSe2+x cubic Pa-3 (205) 6.009 – – – – – 19–1047
Rh3Se8 rhombohedral R-3 (148) 8.490 – 10.196 – – – 85–0731
RhSe2 orthorhombic 20.91 5.951 3.709 – – – metallic(36)
RhTe hexagonal P63/mmc (194) 3.990 – 5.660 – – – 74–0927
RhTe2 cubic Pa-3 (205) 6.441 – – – – – 74–0929 metallic(36)
Rh3Te2 orthorhombic Amam (63) 7.694 12.44 3.697 – – – 71–2190
Rh3Te4 monoclinic I2/m (12) 6.812 3.954 11.23 – 92.55 – 22–1258
Rh3Te8 rhombohedral R-3 (148) 6.425 – – 90.724 – – 71–0416
IrS2 orthorhombic Pnam (62) 19.79 5.624 3.567 – – – 85–2398 ∼ 0.9 (opt)(27)
Ir2S3 orthorhombic Pbcn (60) 8.487 6.019 6.169 – – – 45–1034
IrSe2 orthorhombic Pnam (62) 20.95 5.938 3.742 – – – 85–2399 ∼ 1.0 (opt)(27)
IrTe2 hexagonal P3m1 (156) 3.930 – 5.386 – – – 50–1555 metallic(27)
Ir3Te8 cubic Pa-3 (205) 6.414 – – – – – 73–1681 metallic(35)

  • ME2 (RhS2, RhSe2, RhTe2, IrS2, IrSe2, IrTe2)

  • M2E3 (Rh2S3, Rh2Se3, Ir2S3)

  • Rh3E4 (E = S, Se, Te), and

  • M3E8 (Rh3Se8, Rh3Te8, Ir3Te8).

Additionally, other rhodium chalcogenide stoichiometries have been isolated and these include Rh17S15, RhSe2+x and RhTe, Rh3Te2.

Rhodium and iridium chalcogenides are usually prepared by sintering (at 650–1100°C) pressed stoi-chiometric mixtures of the constituent elements in evacuated sealed ampoules (27–35). Single crystals, for example, Rh2S3, are prepared by chemical transport techniques (for example, using bromine as a transport agent) (29) and tellurium flux techniques (for example, for Ir3Te8) (36).

Rhodium and iridium chalcogenides are usually diamagnetic, with a few exceptions, such as Rh3S4, which shows temperature-independent paramagnetism (35). The compounds display metallic to semiconducting behaviour (28, 33, 35–40).

The structures of these compounds have been established by powder as well as by single crystal X-ray diffraction methods. The compounds show diverse structural preferences, such as pyrite-type, CdI2-type or NiAs structures. For instance, MTe compounds (M = Rh or Ir) exist in a NiAs structure while ME2 compounds adopt pyrite- and CdI2-type structures, see Figure 2 (28, 31, 32, 34, 40–42). Although RhS2 has been reported, it seems that the pure compound does not exist as attempts to prepare it have resulted in the formation of Rh2S3, RhS3 and other phases (28). RhTe2 adopts the pyrite structure, see Figure 2, but does not exist as a stoichiometric compound above 550°C (41).

Fig. 2

Structures of IrTe2 (CdI2 system) (above) and RhTe2 (pyrite system) (right) (41)

IrTe2

IrTe2 shows polymorphism with three phases:

  • 3D polymeric 2D-derived CdI2-type (h-IrTe2 because of its hexagonal cell)

  • pyrite-type (c-IrTe2 because it is cubic), and

  • monoclinic (m-IrTe2) (28, 41).

These three phases and four hypothetical phases (ramsdellite-type, pyrolusite-type, IrS2-type and marcasite-type) have been analysed by extended Huckel tight-binding electronic band structure calculations (34, 43). Monoclinic IrTe2 (m-IrTe2) shows structural features of both CdI2 and pyrite-type IrTe2 (c-IrTe2) phases (34). The high pressure behaviour of h-IrTe2 was studied at up to 32 GPa at room temperature and two new forms were obtained (44). The first structural transition took place at ∼ 5 GPa and led to the monoclinic form (m-IrTe2). The second transition occurred at 20 GPa and gave rise to the cubic pyrite phase (c-IrTe2) (44).

IrSe2

The structure of IrSe2 is similar to the marcasite system (42), as are the structures of IrS2 and the low-temperature modification of RhSe2 (a = 20.91 (3), b = 5.951 (6), c = 3.709 (4) Å) (28). Each iridium atom of IrSe2 is located at the centre of a distorted octahedron with three Se neighbours at a distance of 2.44 Å and three others at 2.52 Å. Half of the Se atoms are tetrahedrally surrounded by three iridium atoms and one selenium atom, the Se–Se distance being 2.57 Å. The other half of the Se atoms have a similar neighbourhood, but adjacent Se atoms are spaced at 3.27 Å (42).

M2E3

The structures of M2E3 (Rh2S3, Rh2Se3, Ir2S3) are isomorphous (29) and the metal atoms adopt an octahedral configuration. Every octahedron shares a common face with another octahedron to form octahedron pairs. These pairs are arranged in layers of stacking sequence ABABA…. Four metal atoms surround each chalcogen atom at the vertices of a distorted tetrahedron.

M3E8

The compounds M3E8 (Rh3Se8, Rh3Te8, Ir3Te8) exist in a pyrite-type structure (41, 45–47) and crystallise with rhombohedral symmetry. The structure is a three-dimensional network made of interlinked E2 pairs, see Figure 3.

Fig. 3

Structure of Rh3Te8 (41)

Rh3E4

Rh3E4 (E = S, Te) adopts the NiAs structure and crystallises in a monoclinic system (35). The structure of Rh3S4 consists of edge sharing RhS6 octahedrons which are connected by S2 pairs (S–S = 2.20 Å). The molecule contains Rh6 cluster rings in a chair conformation with the Rh–Rh single bond length of 2.70 Å. Both fragments are linked by common S atoms (35). Structures of Rh17S15 (48) and Rh3Te2 (49) have also been established by single crystal X-ray structural analysis.

Palladium and Platinum Chalcogenides

Palladium and platinum form a wide variety of chalcogenides. They are prepared by heating the required amounts of two elements in an evacuated sealed tube. The material thus obtained is made into powder and annealed at various temperatures (50), often for several days (as in the synthesis of PtS and PtS2) (51, 52). The palladium-sulfur (53), palladium-selenium (50), palladium-tellurium (54), platinum-selenium (55) and platinum-tellurium (56, 57) systems have been investigated by differential thermal analysis and X-ray powder diffraction methods.

In the palladium-tellurium system at least eight binary phases (PdTe, PdTe2, Pd3Te2 Pd7Te3, Pd8Te3, Pd9Te4, Pd17Te4 and Pd20Te7 have been identified and characterised by X-ray diffraction (54). The platinum-tellurium system on the other hand exhibits only four binary phases (PtTe, PtTe2, Pt2Te3 and Pt3Te4) (57, 58). These compositions are constant and there is no appreciable compositional range. The Pt2Te3 is stable up to ∼ 675°C whereas Pt3Te4 melts above 1000°C. The band structures of some of these chalcogenides (PtS (58), PtS2 (59, 60), PtSe2 (60)) have been determined by first principle electronic structure calculations. Semiconducting behaviour for some of these compounds has been noted (58, 60–62).

The binary palladium and platinum chalcogenides show a higher diversity of structures than found for the Rh/Ir and Ru/Os compounds. Besides several other binary phases, see Table III, four general families have been isolated:

Table III

Crystallographic Information and Band Gaps of Palladium/Platinum Chalcogenides

MxEy Crystal system Space group Cell parameters JCPDS-ICDD # File No. Band gap, eV
a, Å b, Å c, Å α, ° β, ° γ, °
PdS tetragonal P42/m (84) 6.429 – 6.611 – – – 78–0206 ∼ 2.0
orthorhombic Pbca (61) 5.460 5.541 7.531 – – – 72–1198
Pd16S7 cubic I-43m (217) 8.930 – – – – – 75–2228
Pd2.8S cubic P 8.69 – – – – – 10–0334
Pd3S orthorhombic Ama2 (40) 6.088 5.374 7.453 – – – 73–1831
Pd4S tetragonal P-421C 5.114 – 5.590 – – – 73–1387
PdSe tetragonal P42/m (84) 6.711 6.895 – – – 18–0953
Pd17Se15 cubic Pm3m (221) 10.60 – – – – – 73–1424
Pd7Se4 orthorhombic P21212 (18) 5.381 6.873 10.172 – – – 44–0875
Pd2.5Se – – – – – – – – 11–0499
Pd3Se – – – – – – – 44–0876
Pd34Se11 monoclinic P21/c (14) 21.41 5.504 12.030 – 99.440 – 79–0141
Pd7Se monoclinic P21/c (14) 9.462 5.354 5.501 – 86.50 – 44–0877
Pd4Se tetragonal P-421c (114) 5.232 – 5.647 – – – 73–1386
Pd4.5Se tetragonal P-421c (114) 4.460 – 5.394 – – – 44–0877
Pd8Se – – – – – – – – 49–1709
PdSe2 orthorhombic Pbca (61) 5.741 5.866 7.691 – – – 72–1197
PdTe hexagonal P63/mmc (194) 4.152 – 5.670 – – – 29–0971
Pd3Te2 orthorhombic Amcm (63) 7.900 12.68 3.856 – – – 43–0813
PdTe2 hexagonal P-3m1 (164) 4.036 – 5.126 – – – 88–2279
Pd9Te4 monoclinic P21/c (14) 7.458 13.93 8.839 – 91.97 – 35–1013
Pd2.5Te – – – – – – – – 11–0452
Pd3Te – – – – – – – – 11–0451
Pd2Te – – – – – – – – 11–0450
Pd20Te7 rhombohedral R-3 (148) 11.79 – 11.172 – – – 43–0810
Pd8Te3 orthorhombic – 12.84 15.12 11.304 – – – 43–1293
Pd7Te2 monoclinic – 7.444 13.91 8.873 – 92.46 – 43–1294
Pd7Te3 monoclinic – 7.44 13.92 8.87 – 92.46 – 43–1294
Pd4Te cubic F-43m (216) 12.67 – – – – – 11–0449
Pd17Te4 cubic F-43m (216) 12.65 – – – – – 43–1292
PtS tetragonal P42/mmc (131) 3.470 – 6.109 – – – 88–2268 ∼ 1.41
PtS2 hexagonal P-3m1 (164) 3.543 – 5.038 – – – 88–2280
PtSe2 hexagonal P-3m1 (164) 3.727 – 5.031 – – – 88–2281
Pt5Se4 monoclinic P21/c (14) 6.584 4.602 11.10 – 101.6 – 45–1466
PtTe monoclinic C2/m (12) 6.865 3.962 7.044 – 108.98 – 88–2275
PtTe2 hexagonal P-3m1 (164) 4.025 – 5.220 – – – 88–2277
Pt3Te4 rhombohedral R-3m (166) 3.988 – 35.39 – – – 88–2264
Pt2Te3 rhombohedral R-3m (166) 4.003 – 50.89 – – – 88–2263
Pt4Te5 – – – – – – – – 38–0900
Pt5Te4 – – – – – – – – 38–0899

  • ME (PdS, PdSe, PdTe, PtS, PtSe, PtTe)

  • Pd3E, (E = S, Se, Te)

  • Pd4E (E = S, Se, Te), and

  • ME2 (PdS2, PdSe2, PdTe2, PtS2, PtSe2, PtTe2).

Single crystal X-ray analysis of PdSe shows that there are three crystallographically unique Pd atoms, each site being in a slightly distorted square-planar environment. Each of the two crystallographically independent Se atoms is coordinated by a distorted tetrahedron of Pd atoms (63).

PdTe crystallises in the NiAs structure and can be modelled as a single h.c.p. lattice.

PdS2 and PdSe2 exist in a deformed pyrite-type structure (50), while the remaining ME2 adopt a CdI2 structure. PdTe2 has a layered structure with the layers stacking along the (001) direction, see Figure 4. The Pd4+ cations are octahedrally coordinated. The layers are formed by octahedra sharing edges along the [100], [010] and [110] directions. The Pd–Te bond distance is 2.693 (2) Å (64).

Fig. 4

Structure of PdTe2 (64)

Although the single crystal X-ray structure of Pd17Se15 can be analysed in any of the three space groups, viz. Pm3m, P43m and P432, refinement based on the former space group gives the lowest standard errors (65). There are four crystallographically different palladium atoms see Figure 5. One of the palladium atoms has a regular octahedron of selenium atoms with Pd–Se distances of 2.58 Å. The remaining three palladium atoms are coordinated each with four selenium atoms either in a flattened tetrahedron (one Pd centre) with average Pd–Se distances of 2.48 Å or square plane (two Pd atoms) with Pd–Se distances of 2.53 and 2.44 Å. The square planar palladium atoms are also coordinated to palladium atoms with Pd…Pd distance of 2.78 Å.

Fig. 5

Structure of Pd17Se15 (65)

The Use of Platinum Group Metal Chalcogenides in Catalysis Hydrodesulfurisation

Several platinum group metal sulfides, particularly RuS2, have been extensively employed as catalysts for hydrodesulfurisation (HDS) reactions (66–79). It has been shown that semiconducting transition metal sulfides, such as PdS, PtS, Rh2S3, Ir2S3, RuS2, have higher catalytic activity than the metallic sulfides (66). They have been used, supported on γ-Al2O3 or carbon, or as bulk catalysts, for HDS of several thiophene derivatives, such as thiophene, 3-methylthiophene, benzothiophene, dibenzothiophene or 4,6-dimethyldibenzothiophene, see Equation (i)..

In RuS2 the three coordinate surface Ru atoms, such as those found on the (111) surface, appear to provide active sites for HDS (77). The temperature programmed desorption profiles indicate that two different adsorbed species that have different relative concentrations are a factor for the degree of reduction caused by RuS2. NMR results suggest that one of the species leads to the formation of SH groups while the other species has hydridic character (80). To study the effect of the surface Ru–S coordination number on the surface S–H and Ru–H species for thiophene adsorption on RuS2, a topology study of the Laplacian of electron density of selected (100) and (111) surfaces was carried out (81, 82). Acidic Lewis and Brønsted sites are created in mild reducing conditions. The Lewis acidic sites play an important role in activating sulfur-containing molecules and subsequently in their transformations. Hydrogenation properties are related to Ru sites with a low S coordination (83).

Thiophene adsorption on stoichiometric and reduced (100) surfaces of RuS2 has been studied using ab initio density functional molecular dynamics. On the stoichiometric RuS2 surface, thiophene is adsorbed in a tilted η1 position where the sulfur atom of the thiophene molecule forms a bond with the surface Ru atom similar to that in bulk RuS2; but there is no activation of the molecule. The formation of sulfur vacancies on the surface creates a chemically active surface and the possibility for thiophene adsorption in the η2 position when the thiophene molecule is activated (84, 85). Scattered-wave calculations on model catalyst clusters and catalyst-thiophene (or related compounds) systems have indicated that pπ bonding between the S atoms of the catalyst and the S and C atoms of thiophene is responsible for binding the thiophene molecule to the catalyst in the initial stages of the HDS process (86). Electronic and bonding properties of the RuS2 and related thiophene adsorption systems have also been studied by discrete vibrational-Xα calculations (87).

The cleavage of the sp2 carbon-heteroatom bond in the hydroprocessing of substituted benzenes, such as aniline, phenol, diphenylsulfide, chlorobenzene, over unsupported transition metal sulfides at 250°C and 70 bar H2 pressure was studied. Hydrogenolysis of the sp2 carbon-substituent bond results from attack by a soft nucleophile, such as an hydride ion, on the carbon bearing the substituent (88).

Hydrodenitrogenation

The activity of a carbon-supported metal sulfide catalyst in the hydrodenitrogenation (HDN) of quinoline increases in the order Ni < Pd < Pt (89). Platinum group metal chalcogenides have been employed in HDN reactions (89–93) and the activity is related to the acidic-basic properties of the active phase. The best catalyst appeared to have a good balance between the acidic-basic properties responsible for C–N and C–C bond cleavage and labile superficial S anions, which, in a reducing atmosphere, leads to a large number of active sites for hydrogenation reactions (93). Ruthenium and rhodium sulfides gave only a low conversion of quinoline to hydrocarbons (propylbenzene and propylcyclohexane) (89), while the hydrodenitrogenation of quinoline, decahydroquinoline, cyclohexylamine and o-propylamine over Rh and Ir sulfide catalysts was shown to lead to the formation of hydrocarbons (90, 94).

Hydrotreatment of naphtha, which contains mainly nitrogen (pyridines, anilines and quinolines), sulfur- and oxygen-containing heteroatom compounds (92), has been carried out on transition metal sulfides which were used for the removal of nitrogen compounds.

Hydrogenation Reactions

Platinum group metal chalcogenides have also found use as catalysts in hydrogenation reactions (95–102). Sulfides (PdS2, Ir2S3, OsS2 (95)) selenides (Ru2Se3 (96)) and tellurides of Rh, Pd and Pt have been used for the reduction of nitrobenzene to aniline in 95–99% yield. The catalysts are insensitive to sulfur poisoning and are active at low temperatures and low hydrogen pressure. The reductive alkylation of aniline and substituted anilines on Ru, Rh, Pd or Pt selenides/tellurides has also been carried out (96). For instance, aniline was converted to isopropylaniline in the presence of ruthenium selenide (Ru2Se3) catalyst, see Equation (ii) (96):

Palladium sulfide catalysts are active for the hydrogenation of thiophenes (thiophene, 2-methylthiophene and benzothiophene) to tetrahydrothiophenes (Equation (iii), (97–101).

In these reactions hydrogenation and hydrogenolysis often proceed simultaneously. Thus, the hydrogenation of thiophene yields thiolane and the hydrogenolysis products, butane and H2S, which are formed during the decomposition of thiophene and thiolane (97). PdS supported on aluminosilicate showed higher activity (by 1 to 2 orders of magnitude) than Rh, Ru, Mo, W, Re, Co and Ni sulfides (101).

Pyridine hydrogenation to piperidine has been investigated using ruthenium sulfide supported on Y-zeolite or alumina catalysts (103).

RuS2 supported on a dealuminated KY-zeolite showed very high activity (roughly 300 times that of an industrial NiMo/Al2O3 hydrotreating catalyst) for the hydrogenation of naphthalene to tetralin (104).

Both palladium sulfides and platinum sulfides have been employed for hydrogenation reactions, such as the hydrogenation of a gasoline pyrolysis residue over Pd sulfide/Al2O3 (105), and the hydrogenation of naphthalene to tetralin over carbon-supported Pd or Pt sulfide (78).

The hydrogenation of diethyldisulfide gives ethanethiol with selectivity > 94% at atmospheric pressure in the presence of supported transition metal sulfide catalysts (102). Bimetallic catalysts were less active than monometallic compounds in this reaction. Both bulk and SiO2 supported palladium sulfides have been employed for the production of methanol from the hydrogenation of CO (106, 107).

Isomerisation and Acetoxylation Reactions

Solid bed catalysts containing Pd/Se or Pd/Te on a SiO2 carrier have been employed for the isomerisation of alkenes, particularly 3-butene-1-ol (108). Besides isomerisation, Pd/Te or Pt/Te supported on SiO2 have been used to prepare unsaturated glycol diester compounds by treating a conjugated diene (such as butadiene) with a carboxylic acid (such as acetic acid) in the presence of oxygen (109). This process, see Equation (iv), has been industrialised by Mitsubishi Kasai Corp. for the production of 1,4-butanediol from 1,4-butadiene (110). The production of unsaturated glycol diesters (such as 1,4-diacetoxy-2-butene and butanediols) comprises reacting a conjugated diene with the carboxylic acid and O2 in the presence of a solid Rh-Te catalyst (111). A Rh2Te catalyst (∼ 3%) on activated charcoal has been used for the acetoxylation in AcOH of 1,3-cyclopentadiene to diacetoxycyclopentane (112).

Dehydrogenation Reactions

Platinum group metal chalcogenides have found use in dehydrogenation reactions. The dehydrogenation of tetrahydrothiophene over RuS2 yields thiophene in 92% yield (Equation (iii)) (113). The active sites for the hydrogenation and hydrodesulfurisation are anionic vacancies in the sulfide catalysts (RuS2). The higher lability of S22−anions relative to S2− anions in a reducing atmosphere explains the higher activity of RuS2 (93). The catalytic dehydrogenative polycondensation of 1,2,3,4-tetrahydroquinoline using transition metal sulfides (PdS, PtS, RuS, RhS2) provides a direct route to the synthesis of unsubstituted quinoline oligomers. RuS gives a maximum yield of 97% (114, 115).

The photocatalytic decomposition of H2O into H2 and O2 takes place over RuS2 powder as well as over RuS2 supported on various substrates (such as SiO2, zeolites) under UV irradiation (116–118).

Platinum Group Metal Chalcogenides as Semiconductors and in the Electronic Industry

Platinum group metals (particularly Pd and Pt) are used for low resistance ohmic contacts in semiconducting electronic devices. For device reliability thermodynamically stable contacts are very important. Therefore reactions occurring at the interface between the metal contacts and the II-VI semiconductors have been extensively studied in recent years (119–133). For instance, it has been found that at the interface of Pt/CdTe diffusion couples a nonplanar reaction layer of the intermetallics CdPt and PtTe is formed (121).

The crystallographic microstructure and electrical characteristics of platinum group metals (mainly Pd) and Ni or Au ohmic contacts on ZnSe and on p-type (001) ZnTe layers have been investigated as a function of annealing temperature. The specific contact resistance of these contacts depends strongly on annealing temperature and the palladium layer thickness (122).

In Pd/ZnSe, palladium forms a ternary epitaxial phase Pd5+xZnSe at 200°C which is stable up to 450°C while platinum begins to form Pt5Se4 at 575°C at the Pt/ZnSe interface (119).

PdS and PtS have been employed as light image receiving materials with silver halides (134, 135). Photographic film containing 6–10 mol m−2 PdS coating gives dense black images with high contrast (136). In optical disc recording films PdTe2 is one of the active components (137). Palladium sulfide has also been used for lithographic films (138, 139) and lithographic plates with high resolution (140, 141). Semiconducting films of metal sulfide polymer composites were obtained when organosols of PdS in DMF were prepared from Pd(OAc)2 and H2S, and followed by addition of polymers (142).

Thin films of PdS and PtS have been deposited on GaAs substrate from [M{S2CNMe(c-Hex)}2] (M = Pd or Pt) by low pressure MOCVD (143). The complexes also serve as precursors for the growth of nanocrystals of PdS and PtS which are formed by thermolysis of the complex in trioctylphosphine oxide (143).

PdS thin films have been deposited onto Si and quartz substrates at 10−2 torr from a single source precursor [Pd(S2COPri)2]. Two different vapour deposition processes: photochemical (308 nm laser irradiation) and thermal (350°C) were employed (144). The PdS films are in the polycrystalline tetragonal phase. Palladium sulfide in a polymer matrix has been used for the manufacture of semiconductors and solar cells (145). Semiconducting and photoelectrochemical properties of PdS have also been investigated (146).

PtS2 nanoclusters synthesised from PtCl4 and (NH4)2S in inverse micelles show an indirect band gap of 1.58 eV as compared to 0.87 eV for bulk PtS2. Nanoclusters with double the mass show a band gap of 1.27 eV (147).

Aqueous dispersions of PdS particles have been prepared from PdCl2 or Na2PdCl4 with Na2S solutions. Uniform spherical particles of diameter 20–30 nm were obtained in acidic medium in the presence or absence of surfactants. Surfactants of the AVANEL S series enhanced deposition of PdS particles on an epoxy circuit board (148).

Thin films (0.05–1 μm thick) of pyrite-type RuS2 (polycrystalline or epitaxial) have been grown on various substrates such as silica, sapphire and GaAs, by MOCVD using ruthenocene and H2S as precursors (149). The films have been characterised by X-ray diffraction, microprobe and SIMS analysis, and electrical and optical measurements.

The BARC group has recently designed several molecular precursors for the synthesis of palladium chalcogenides (150–154). The compound [Pd(Spy){S2P(OPri)2}(PPh3)] containing a chelating dithiophosphate group (Figure 6) undergoes a three-stage decomposition leading to the formation of PdS2 at 357°C (150). The dimeric methyl- allyl palladium complexes, [Pd2(μ-ER)2(η3-C4H7)2] afford polycrystalline Pd4E (E = S or Se), see Scheme, and amorphous Pd3Te2, the former at moderately low temperatures (refluxing xylene) and the latter at room temperature (151). Thermo- gravimetric analysis of [PdCl(SeCH2CH2NMe2)]3 and [PdCl(SeCH2CH2NMe2)(PR3)] (PR3 = PPh3 or Ptol3) reveals that these compounds undergo a two-step decomposition leading to polycrystalline Pd17Se15 (153). The thermogravimetric analysis of [PdCl{Te(3-MeC5H3N)}(PPh3)], see Figure 7 shows that the compound decomposes in a single step at 290°C to give PdTe (by XRD, Figure 8) as aggregates of microcrystals (by SEM, Figure 9) (154).

Fig. 6

Molecular structure with atomic numbering scheme for (Pd(Spy)[S2P(OPri)2)(PPh3)] (150)

Fig. 7

Structure of [PdCl{Te(3-MeC5H3N)}(PPh3)] (154)

Fig. 8

XRD pattern of PdTe obtained from [PdCl{Te(3-MeC5H3N)}(PPh3)] (154)

Fig. 9

SEM picture of PdTe obtained from [PdCl{Te(3-MeC5H3N)}(PPh3)] (154)

Scheme

Conclusions

The great structural diversity and catalytic applications (HDS, HDN, hydrogenation, etc.) of platinum group metal chalcogenides, seem to offer new avenues for further research. Clear trends seem to be emerging in molecular precursor chemistry for the preparation of metal chalcogenides. It is believed that such trends would provide opportunities to isolate not only known stable or metastable phases at low temperatures (for instance palladium allyl complexes (151)) but also as yet unknown stoichiometries. It is hoped that this brief review will serve as an interface between scientists working in areas such as mineralogy, catalysis, materials science and solid state structural chemistry.

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Acknowledgements

We thank Drs J. P. Mittal and S. K. Kulshreshtha for encouragement of this work. Also, many thanks for permission to reproduce some of the Figures in this article.

The Authors

Sandip Dey is a Scientific Officer in Novel Materials and Structural Chemistry at Bhabha Atomic Research Centre (BARC), Mumbai, India. He has an MSc (1996) from Burdwan University and was selected for the “Advanced Course on Chemical Sciences and Nuclear Sciences” of BARC. He has a PhD (2003) from Mumbai University (Jain). His interests are the chemistry of platinum chalcogenolates and NMR spectroscopy.

Vimal K. Jain is Head, Synthesis and Pure Materials Section, Novel Materials and Structural Chemistry at BARC. He has an MSc (1976) from Agra University and PhD (1981) from Rajasthan University, and was a Post Doctoral Fellow at the University of Guelph, Canada. In 1984 he was appointed as Scientific Officer in the Chemistry Division, BARC. His research interests include inorganic and organometallic chemistry of the platinum group metals and main group elements, design and development of molecular precursors for advanced inorganic materials, and multinuclear NMR spectroscopy.

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